A variety of mass spectrometry methods have been developed. Typically in such methods, trapped ions confined within a trapping field are ejected from the field for detection by an external detector.
For example, in the conventional mass spectrometry techniques known as "MS/MS" methods, ions (known as "parent ions") having mass-to-charge ratio within a selected range are stored in an ion trap. The trapped parent ions are then allowed, or induced, to dissociate to produce ions known as "daughter ions." The daughter ions are then ejected from the trap and the ejected daughter ions are detected.
For example, U.S. Pat. No. 4,736,101, issued Apr. 5, 1988, to Syka, et al., discloses an MS/MS method in which daughter ions ejected from a quadrupole ion trap are detected by an electron multiplier detector positioned outside the trap.
In another conventional mass spectrometry technique known as a chemical ionization or "CI" method, as described for example in U.S. Pat. No. 4,686,367, issued Aug. 11, 1987, to Louris, et al., stored reagent ions are allowed to react with analyte molecules in a quadrupole ion trap. The trapping field is then scanned to eject product ions which result from the reaction, and the ejected product ions are detected outside the trap.
Techniques for detecting trapped ions within an ion trap have been suggested. For example, the article by Fulford, et al., entitled "Radio-Frequency Mass Selective Excitation and Resonant Ejection of Ions in a Three-Dimensional Quadrupole Ion Trap," J. Vac. Sci. Technol., 17(4), 1980, pp. 829-835, discloses (at page 830) a "resonant power absorption" technique for indirect, in-trap detection of resonating trapped ions. In this technique, the presence of resonating trapped ions is indirectly detected by monitoring the power absorbed from a voltage signal generator while the generator applies a swept sawtooth signal to the ring electrode of a quadrupole ion trap.
The Fulford, et al. article also discloses (at page 830) an example of another technique for indirect detection of resonating trapped ions (sometimes referred to as an "image current detection" technique). In this technique, a frequency tuned detection circuit is connected across the end electrodes of an ion trap and is balanced when no ions are present in the trap. Then, ions are introduced into the trap, an RF voltage signal is applied to the trap, and the amplitude of the RF voltage signal is slowly swept. The motion of resonating trapped ions is detected as an induced alternating potential (or current) in the frequency tuned detection circuit, each time that the frequency of the ions' secular motion matches that of the tuned circuit.
Conventional methods for indirect, in-trap detection of trapped ions are complicated and difficult to implement in a manner providing adequate sensitivity.
Conventional techniques for detecting ions after they have been ejected from an ion trap also suffer limitations and disadvantages, which result principally from the need to perforate one or more of the trap electrodes to permit the ions to escape from the trap. A perforated trap electrode will inherently block a substantial number of the ions which strike it (although a substantial number may also pass through the perforations), thus inherently limiting the effective sensitivity of an out-of-trap detector which receives the ions that pass through the perforations. Furthermore, the presence of perforations in an otherwise precisely shaped electrode of an ion trap will inherently introduce distortions in the trapping field.